Electrostatic copying systems are known in the prior art in which a photoconductive surface first is charged and then is exposed to a light image of the original to be copied to produce a latent electrostatic image on the photoconductor. This image is subjected to a developer made up of electrostatic toner particles in a carrier. In the course of development, the toner particles are attracted to the charged areas to develop the latent image.
There are known in the prior art organic photoconductive materials which combine the advantages of transparency, flexibility, ease of coating and panchromatic response with low cost. Kaale & Company of Germany has developed a large number of organic photoconductors such as the oxazoles described in Uhlig German Pat. No. 1,117,391 of Nov. 16, 1961. Many other organic photoconductors are known such as benzanthrones, triazines, acylhydrazones and the like.
While organic photoconductors of the type mentioned above combine many advantages, their use heretofore has been limited owing to the fact that their discharge times are inordinately long. That is to say, in use of such materials in electrostatic copying, they may be initially charged to, for example, 800 volts. Tests have shown that an initial discharge from about 800 volts to about 100 volts requires about 3 footcandle-seconds. However, to discharge the photoconductor to about 10 volts requires 36 footcandle-seconds. Complete discharge of the photoconductor requires an inordinately great exposure of the material. To achieve such a discharge, the exposure time would have to be so long, or the optical system so large, or both, as to make a system of electrostatic copying employing an organic photoconductor entirely impracticable. That is to say, if, using an organic photoconductor, an electrostatic copying machine were run at conventional speeds with a conventional exposure system, development would result in a copy in which desired image areas could scarcely be distinguished from background areas.
Recognizing the problem of employing organic photoconductors in an electrostatic copying system, we conceived of the application of an electrical field to counteract the effect of background potential in the organic photoconductor while at the same time adequately developing desired image areas. In setting up such a counter field, we apply a bias potential between a conductive substrate carrying the organic photoconductor and a conductive developer applicator electrode.
While application of the counter field in the manner described above initially successfully overcame background potential in the photoconductor, it resulted in collection of toner particles on the electrode with the result that the system ultimately became ineffective and the quality of copies produced rapidly deteriorated. We provide our system with means for continuously cleaning the biasing electrode to remove toner particles which otherwise would collect thereon under the influence of the biasing potential.
In the course of our investigation we further noted the presence of "tails" at the trailing edge of each toner image deposit. We discovered that this phenomenon, which can be termed "toner-tank tailing," can be substantially eliminated by producing over the image area being developed a flow of developer at the surface speed of the photoconductor relative to the developer station.